The number of cellular phones being used in the United States is growing at a rapid rate. This means that more and more people are carrying cellular phones with them from one location to another. As such, most cellular phone users desire a phone that is lightweight, compact, easy to carry and full of features. As a result of this demand, cellular phone designers are striving to develop smaller types of cellular phones that are still capable of incorporating all of the features that are available to cellular phone users.
Most cellular phones use an LCD to communicate information to the user while the phone is being used. A LCD uses strips or squares of liquid-crystal material to form digits or pixels that communicate information when energized. Some aspects of the manufacture of LCDs are extremely precise. For example, the tolerances for the location of the electrical traces or contacts on the LCD that are used to drive the LCD are on the order of .+-.0.025 mm. The electrical traces that interconnect the LCD with a second device are typically made from indium tin oxide ("ITO") or some other conductive material. However, other aspects of the LCDs are not as precise due to the nature of the materials that are used or the methods in which they are manufactured.
LCDs have a very wide tolerance for the mechanical dimensions of the glass and the positioning of the electrical traces with respect to the edges of the glass. These tolerance ranges are much larger and are generally only required to be accurate within .+-.0.200 mm or even greater. This large tolerance is predominantly a result of the manufacturing process that necessitates scoring and breaking of a larger glass panel to create discrete LCDs.
Due to the number of parts and their associated tolerances, designing mechanical interconnects for the LCDs that are used in cellular phones is problematic. This is particularly true for LCDs that use chip-on-glass technology, in which an LCD driver chip is bonded directly to the LCD glass. These designs do not incorporate a secondary connection means, such as a flex film, which is used in chip-on-flex and chip-on-tab technologies. Flex film is an array of conductors bonded to a thin dielectric film. In these designs, one end of the flex film is bonded to the LCD driver chip while the other end, because the flex film is flexible, may be manipulated and bent around objects until being connected to a circuit which drives the LCD driver chip. However, using flex film is no longer appealing in cellular phone designs due to size constraints.
In prior cellular phone designs, the LCD is generally held in place with a light guide. The LCD is mounted in the lightguide and the edges of the lightguide are used to align and hold the LCD in position. Thus, the edges of the LCD, which have wide tolerances, are used to align the LCD in place. In these devices, an LCD connector is retained by the lightguide and used to electrically connect the LCD with the printed circuit board of the cellular phone.
Since all known prior art cellular phone designs align the LCD by using the edge of the LCD glass they suffer from the large tolerances associated with the LCD glass. To that end, this creates connection and quality problems that are difficult to deal with in the manufacture of a larger number of cellular phones.
Since the LCD driver chip must be electrically connected with the contacts on the printed circuit board of the cellular phone an optimization of the respective interconnection between the of the LCD contacts and the printed circuit board contacts is required to provide a reliable and repeatable mechanical design. Best engineering practice dictates that the system must function under all tolerance conditions. However, since the LCD is captured and aligned by the edges of the LCD glass, ultimately the large glass tolerances drive the design of these devices to incorporate a large connection pad size and pitch, often beyond the limits of practicality. This hinders efforts to miniaturize the size of cellular phones.
Another problem that must be dealt with is that the performance and functionality of the LCD may be negatively impacted due to the relatively high resistance of the indium-tin oxide which is used as the material to electrically connect the driver to the pixels within the LCD. The ITO traces therefore need to be minimized for low resistance and optimal performance. This is in direct conflict with the requirements of large ITO pads required for reliable design. To illustrate, the acknowledge pulse of the typical LCD may not work if the connection resistance approaches 90 m.OMEGA., which allows for very little ITO in the electrical path.
As cellular phones get smaller and more featured, space within the transceiver has become extremely important. The problem associated with interconnecting the LCD with the cellular phone's printed circuit board was not dealt with in prior devices because designs were either accommodating to the large tolerances or design principles were sacrificed to meet space requirements. To that end, a need exists for a way to optimally electrically interconnect the LCD to the printed circuit board in a cellular phone with precision.